1. Field of the Invention
The present invention relates to a mirror driving device and a driving method thereof, and more particularly to a structure of a micromirror device suitable for an optical deflector used for optical scanning and a driving method thereof.
2. Description of the Related Art
A microscanner fabricated using a silicon (Si) microfabrication technology (hereinafter referred to as “microelectromechanical system (MEMS) scanner”) is characterized by its small size and low power consumption, and is thus expected to be widely used in applications ranging from a laser projector to an optical diagnostic scanner such as an optical coherence tomograph.
There are various driving systems for MEMS scanners. Among these, a piezoelectric driving system which uses the deformation of a piezoelectric body is regarded as having a higher torque density and a smaller size and obtaining a higher scan angle compared to other methods, and is thus considered to be promising. Particularly, in applications requiring a high displacement angle such as in a laser display, resonance driving is mainly used, and at this time, the height of a torque of the piezoelectric driving system is a great advantage.
As a piezoelectric MEMS scanner in the related art, for example, as described in JP2009-2978A, there is a system in which a torsion bar is connected to a connection part (joining part) in an actuator having a structure in which two cantilevers are connected, and the torsion bar is caused to undergo tilt displacement by driving the cantilever in antiphase (JP2009-2978A).
In addition, as in Optical MEMS and Their Applications Conference, 2006, IEEE/LEOS International Conference on, 2006, 25-26, and Japanese Journal of Applied Physics, The Japan Society of Applied Physics, 2010, 49, 04DL19, there may be cases where an actuator has a circular or elliptical shape. By causing the actuator to have such a shape, the length of the actuator can be increased compared to a linear cantilever, so that the displacement amount can be increased. In the structures of JP2009-2978A, Optical MEMS and Their Applications Conference, 2006. IEEE/LEOS International Conference on, 2006, 25-26, and Japanese Journal of Applied Physics, The Japan Society of Applied Physics, 2010, 49, 04DL19, two plate-like actuators disposed on both sides of the rotation axis of a mirror are provided, and the actuators are common in that base end parts which are separated from each other in a direction perpendicular to the rotation axis are fixed.
Contrary to this, JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, Vol. 21, 6 (2012), 1303-1310 proposes a structure in which two plate-like actuators disposed on both sides of the rotation axis of a mirror are provided and the actuators are fixed on the rotation axis of the mirror. This structure has an advantage that a mirror tilt angle that is obtained during resonance driving is large because the amount of the actuator displaced during static driving is larger than that of the structure of JP2009-2978A.
However, in the piezoelectric MEMS scanner having such a structure, the piezoelectric torque cannot be efficiently converted into tilt displacement, and a high voltage of about 25 V is necessary to obtain a sufficient displacement angle. In consideration of the durability of a lead zirconate titanate (PZT) thin film, driving at about 15 V is preferable.
In addition, in a case of an operation using the resonance driving, in order to maintain vibration in a resonance mode, a sensor (stress detection part) which monitors the drive displacement is necessary. For this, one of the actuators needs to be used as a sensor part, which causes a problem that the driving force significantly decreases to about half.
The present invention has been made taking the foregoing circumstances into consideration, and an object thereof is to provide a mirror driving device and a driving method thereof capable of improving a displacement efficiency compared to a structure in the related art and obtaining a sufficiently large displacement angle even in a case where a sensor part is provided.